table of contents
other versions
- jessie 3.17-1.1
- jessie-backports 4.7.3-1~bpo8+1
- stretch 4.7.3-1
- testing 4.20.1-2
- stretch-backports 4.17-1~bpo9+1+b1
- unstable 4.20.2-1
other sections
BTRFS-MAN5(5) | Btrfs Manual | BTRFS-MAN5(5) |
NAME¶
btrfs-man5 - topics about the BTRFS filesystem (mount options, supported file attributes and other)DESCRIPTION¶
This document describes topics related to BTRFS that are not specific to the tools. Currently covers: 1.mount options
2.filesystem features
3.file attributes
4.control device
MOUNT OPTIONS¶
This section describes mount options specific to BTRFS. For the generic mount options please refer to mount(8) manpage. The options are sorted alphabetically (discarding the no prefix). acl, noacl(default: on)
Enable/disable support for Posix Access Control Lists (ACLs). See the
acl(5) manual page for more information about ACLs.
The support for ACL is build-time configurable (BTRFS_FS_POSIX_ACL) and mount
fails if acl is requested but the feature is not compiled in.
alloc_start=bytes
(default: 1M, minimum: 1M)
Debugging option to force all block allocations above a certain byte threshold
on each block device. The value is specified in bytes, optionally with a K, M,
or G suffix (case insensitive).
This option was used for testing and has no practical use, it’s slated to
be removed in the future.
autodefrag, noautodefrag
(since: 3.0, default: off)
Enable automatic file defragmentation. When enabled, small random writes into
files (in a range of tens of kilobytes, currently it’s 64K) are
detected and queued up for the defragmentation process. Not well suited for
large database workloads.
The read latency may increase due to reading the adjacent blocks that make up
the range for defragmentation, successive write will merge the blocks in the
new location.
Warning
Defragmenting with Linux kernel versions < 3.9 or ≥ 3.14-rc2 as well
as with Linux stable kernel versions ≥ 3.10.31, ≥ 3.12.12 or
≥ 3.13.4 will break up the ref-links of CoW data (for example files
copied with cp --reflink, snapshots or de-duplicated data). This may
cause considerable increase of space usage depending on the broken up
ref-links.
barrier, nobarrier
(default: on)
Ensure that all IO write operations make it through the device cache and are
stored permanently when the filesystem is at it’s consistency
checkpoint. This typically means that a flush command is sent to the device
that will synchronize all pending data and ordinary metadata blocks, then
writes the superblock and issues another flush.
The write flushes incur a slight hit and also prevent the IO block scheduler to
reorder requests in a more effective way. Disabling barriers gets rid of that
penalty but will most certainly lead to a corrupted filesystem in case of a
crash or power loss. The ordinary metadata blocks could be yet unwritten at
the time the new superblock is stored permanently, expecting that the block
pointers to metadata were stored permanently before.
On a device with a volatile battery-backed write-back cache, the
nobarrier option will not lead to filesystem corruption as the pending
blocks are supposed to make it to the permanent storage.
check_int, check_int_data,
check_int_print_mask=value
(since: 3.0, default: off)
These debugging options control the behavior of the integrity checking module
(the BTRFS_FS_CHECK_INTEGRITY config option required).
check_int enables the integrity checker module, which examines all block
write requests to ensure on-disk consistency, at a large memory and CPU cost.
check_int_data includes extent data in the integrity checks, and implies
the check_int option.
check_int_print_mask takes a bitmask of BTRFSIC_PRINT_MASK_* values as
defined in fs/btrfs/check-integrity.c, to control the integrity checker
module behavior.
See comments at the top of fs/btrfs/check-integrity.c for more
info.
clear_cache
Force clearing and rebuilding of the disk space cache if
something has gone wrong. See also: space_cache.
commit=seconds
(since: 3.12, default: 30)
Set the interval of periodic commit. Higher values defer data being synced to
permanent storage with obvious consequences when the system crashes. The upper
bound is not forced, but a warning is printed if it’s more than 300
seconds (5 minutes).
compress, compress=type, compress-force,
compress-force= type
(default: off)
Control BTRFS file data compression. Type may be specified as zlib,
lzo or no (for no compression, used for remounting). If no type
is specified, zlib is used. If compress-force is specified, all
files will be compressed, whether or not they compress well. Otherwise some
simple heuristics are applied to detect an incompressible file. If the first
blocks written to a file are not compressible, the whole file is permanently
marked to skip compression.
Note
If compression is enabled, nodatacow and nodatasum are disabled.
datacow, nodatacow
(default: on)
Enable data copy-on-write for newly created files. Nodatacow implies
nodatasum, and disables compression. All files created under
nodatacow are also set the NOCOW file attribute (see chattr(1)).
Note
If nodatacow or nodatasum are enabled, compression is disabled.
datasum, nodatasum
(default: on)
Enable data checksumming for newly created files. Datasum implies
datacow, ie. the normal mode of operation. All files created under
nodatasum inherit the "no checksums" property, however
there’s no corresponding file attribute (see chattr(1)).
Note
If nodatacow or nodatasum are enabled, compression is disabled.
degraded
(default: off)
Allow mounts with less devices than the raid profile constraints require. A
read-write mount (or remount) may fail with too many devices missing, for
example if a stripe member is completely missing from RAID0.
device=devicepath
Specify a path to a device that will be scanned for BTRFS
filesystem during mount. This is usually done automatically by a device
manager (like udev) or using the btrfs device scan command (eg. run
from the initial ramdisk). In cases where this is not possible the
device mount option can help.
Note
booting eg. a RAID1 system may fail even if all filesystem’s
device paths are provided as the actual device nodes may not be
discovered by the system at that point.
discard, nodiscard
(default: off)
Enable discarding of freed file blocks using TRIM operation. This is useful for
SSD devices, thinly provisioned LUNs or virtual machine images where the
backing device understands the operation. Depending on support of the
underlying device, the operation may severely hurt performance in case the
TRIM operation is synchronous (eg. with SATA devices up to revision 3.0).
If discarding is not necessary to be done at the block freeing time,
there’s fstrim tool that lets the filesystem discard all free
blocks in a batch, possibly not much interfering with other operations. Also,
the the device may ignore the TRIM command if the range is too small, so
running the batch discard can actually discard the blocks.
enospc_debug, noenospc_debug
(default: off)
Enable verbose output for some ENOSPC conditions. It’s safe to use but
can be noisy if the system reaches near-full state.
fatal_errors=action
(since: 3.4, default: bug)
Action to take when encountering a fatal error.
bug
flushoncommit, noflushoncommit
BUG() on a fatal error, the system will stay in
the crashed state and may be still partially usable, but reboot is required
for full operation
panic
panic() on a fatal error, depending on other
system configuration, this may be followed by a reboot. Please refer to the
documentation of kernel boot parameters, eg. panic, oops or
crashkernel.
(default: off)
This option forces any data dirtied by a write in a prior transaction to commit
as part of the current commit, effectively a full filesystem sync.
This makes the committed state a fully consistent view of the file system from
the application’s perspective (i.e., it includes all completed file
system operations). This was previously the behavior only when a snapshot was
created.
When off, the filesystem is consistent but buffered writes may last more than
one transaction commit.
fragment=type
(depends on compile-time option BTRFS_DEBUG, since: 4.4,
default: off)
A debugging helper to intentionally fragment given type of block groups.
The type can be data, metadata or all. This mount option
should not be used outside of debugging environments and is not recognized if
the kernel config option BTRFS_DEBUG is not enabled.
inode_cache, noinode_cache
(since: 3.0, default: off)
Enable free inode number caching. Not recommended to use unless files on your
filesystem get assigned inode numbers that are approaching 264. Normally, new
files in each subvolume get assigned incrementally (plus one from the last
time) and are not reused. The mount option turns on caching of the existing
inode numbers and reuse of inode numbers of deleted files.
This option may slow down your system at first run, or after mounting without
the option.
Note
Defaults to off due to a potential overflow problem when the free space
checksums don’t fit inside a single page.
logreplay, nologreplay
(default: on, even read-only)
Enable/disable log replay at mount time. See also treelog.
Warning
currently, the tree log is replayed even with a read-only mount! To disable that
behaviour, mount also with nologreplay.
max_inline=bytes
(default: min(2048, page size) )
Specify the maximum amount of space, in bytes, that can be inlined in a metadata
B-tree leaf. The value is specified in bytes, optionally with a K suffix (case
insensitive). In practice, this value is limited by the filesystem block size
(named sectorsize at mkfs time), and memory page size of the system. In
case of sectorsize limit, there’s some space unavailable due to leaf
headers. For example, a 4k sectorsize, maximum size of inline data is about
3900 bytes.
Inlining can be completely turned off by specifying 0. This will increase data
block slack if file sizes are much smaller than block size but will reduce
metadata consumption in return.
Note
the default value has changed to 2048 in kernel 4.6.
metadata_ratio=value
(default: 0, internal logic)
Specifies that 1 metadata chunk should be allocated after every value
data chunks. Default behaviour depends on internal logic, some percent of
unused metadata space is attempted to be maintained but is not always possible
if there’s not enough space left for chunk allocation. The option could
be useful to override the internal logic in favor of the metadata allocation
if the expected workload is supposed to be metadata intense (snapshots,
reflinks, xattrs, inlined files).
recovery
(since: 3.2, default: off, deprecated since: 4.5)
Note
this option has been replaced by usebackuproot and should not be used but
will work on 4.5+ kernels.
norecovery
(since: 4.5, default: off)
Do not attempt any data recovery at mount time. This will disable
logreplay and avoids other write operations.
Note
The opposite option recovery used to have different meaning but was
changed for consistency with other filesystems, where norecovery is
used for skipping log replay. BTRFS does the same and in general will try to
avoid any write operations.
rescan_uuid_tree
(since: 3.12, default: off)
Force check and rebuild procedure of the UUID tree. This should not normally be
needed.
skip_balance
(since: 3.3, default: off)
Skip automatic resume of interrupted balance operation after mount. May be
resumed with btrfs balance resume or the paused state can be removed by
btrfs balance cancel. The default behaviour is to start interrutpd
balance.
space_cache, space_cache=v2, nospace_cache
(nospace_cache since: 3.2, space_cache=v2
since 4.5, default: on)
Options to control the free space cache. This affects performance as searching
for new free blocks could take longer if the space cache is not enabled. On
the other hand, managing the space cache consumes some resources. It can be
disabled without clearing at mount time.
There are two implementations of how the space is tracked. The safe default is
v1. On large filesystems (many-terabytes) and certain workloads the
v1 performance may degrade. This problem is addressed by v2,
that is based on b-trees, sometimes referred to as free-space-tree.
Compatibility notes:
ssd, nossd, ssd_spread
•the v2 has to be enabled manually at mount
time, once
•kernel without v2 support will be able to
mount the filesystem in read-only mode
•v2 can be removed by mounting with
clear_cache
(default: SSD autodetected)
Options to control SSD allocation schemes. By default, BTRFS will enable or
disable SSD allocation heuristics depending on whether a rotational or
non-rotational disk is in use (contents of
/sys/block/DEV/queue/rotational). The ssd and nossd
options can override this autodetection.
The ssd_spread mount option attempts to allocate into bigger and aligned
chunks of unused space, and may perform better on low-end SSDs.
ssd_spread implies ssd, enabling all other SSD heuristics as
well.
subvol=path
Mount subvolume from path rather than the toplevel
subvolume. The path is absolute (ie. starts at the toplevel subvolume).
This mount option overrides the default subvolume set for the given
filesystem.
subvolid=subvolid
Mount subvolume specified by a subvolid number
rather than the toplevel subvolume. You can use btrfs subvolume list to
see subvolume ID numbers. This mount option overrides the default subvolume
set for the given filesystem.
Note
if both subvolid and subvol are specified, they must point at the
same subvolume, otherwise mount will fail.
subvolrootid=objectid
(irrelevant since: 3.2, formally deprecated since: 3.10)
A workaround option from times (pre 3.2) when it was not possible to mount a
subvolume that did not reside directly under the toplevel subvolume.
thread_pool=number
(default: min(NRCPUS + 2, 8) )
The number of worker threads to allocate. NRCPUS is number of on-line CPUs
detected at the time of mount. Small number leads to less parallelism in
processing data and metadata, higher numbers could lead to a performance hit
due to increased locking contention, cache-line bouncing or costly data
transfers between local CPU memories.
treelog, notreelog
(default: on)
Enable the tree logging used for fsync and O_SYNC writes. The tree
log stores changes without the need of a full filesystem sync. The log
operations are flushed at sync and transaction commit. If the system crashes
between two such syncs, the pending tree log operations are replayed during
mount.
Warning
currently, the tree log is replayed even with a read-only mount! To disable that
behaviour, mount also with nologreplay.
The tree log could contain new files/directories, these would not exist on a
mounted filesystem if the log is not replayed.
usebackuproot, nousebackuproot
Enable autorecovery attempts if a bad tree root is found
at mount time. Currently this scans a backup list of several previous tree
roots and tries to use the first readable. This can be used with read-only
mounts as well.
Note
This option has replaced recovery.
user_subvol_rm_allowed
(default: off)
Allow subvolumes to be deleted by their respective owner. Otherwise, only the
root user can do that.
FILESYSTEM FEATURES¶
The basic set of filesystem features gets extended over time. The backward compatibility is maintained and the features are optional, need to be explicitly asked for so accidental use will not create incompatibilities. There are several classes and the respective tools to manage the features: at mkfs time onlyThis is namely for core structures, like the b-tree
nodesize, see mkfs.btrfs(8) for more details.
after mkfs, on an unmounted filesystem
Features that may optimize internal structures or add new
structures to support new functionality, see btrfstune(8). The command
btrfs inspect-internal dump-super device will dump a superblock, you
can map the value of incompat_flags to the features listed below
after mkfs, on a mounted filesystem
The features of a filesystem (with a given UUID) are
listed in /sys/fs/btrfs/UUID/features/, one file per feature. The
status of is stored insid the file. The value 1 is for enabled,
0 means the feature was had been enabled at the mount time and turned
off afterwards.
Whether a particular feature can be turned on a mounted filesystem can be found
in the directory /sys/fs/btrfs/features/, one file per feature. The
value 1 means the feature can be enabled.
List of features (see also mkfs.btrfs(8) section FILESYSTEM
FEATURES):
big_metadata
(since: 3.4)
the filesystem uses nodesize bigger than the page size compress_lzo::
(since: 2.6.38)
the lzo compression has been used on the filesystem, either as a mount
option or via btrfs filesystem defrag.
default_subvol
(since: 2.6.34)
the default subvolume has been set on the filesystem
extended_iref
(since: 3.7)
increased hardlink limit per file in a directory to 65536, older kernels
supported a varying number of hardlinks depending on the sum of all file name
sizes that can be stored into one metadata block
mixed_backref
(since: 2.6.31)
the last major disk format change, improved backreferences
mixed_groups
(since: 2.6.37)
mixed data and metadata block groups, ie. the data and metadata are not
separated and occupy the same block groups, this mode is suitable for small
volumes as there are no constraints how the remaining space should be used
(compared to the split mode, where empty metadata space cannot be used for
data and vice versa)
on the other hand, the final layout is quite unpredictable and possibly highly
fragmented, which means worse performance
no_holes
(since: 3.14) improved representation of file extents
where holes are not explicitly stored as an extent, saves a few percent of
metadata if sparse files are used
raid56
(since: 3.9)
the filesystem contains or contained a raid56 profile of block groups
skinny_metadata
(since: 3.10)
reduced-size metadata for extent references, saves a few percent of
metadata
FILE ATTRIBUTES¶
The btrfs filesystem supports setting the following file attributes using the chattr(1) utility: aappend only, new writes are always written at the
end of the file
A
no atime updates
c
compress data, all data written after this
attribute is set will be compressed. Please note that compression is also
affected by the mount options or the parent directory attributes.
When set on a directory, all newly created files will inherit this
attribute.
C
no copy-on-write, file modifications are done
in-place
When set on a directory, all newly created files will inherit this attribute.
Note
due to implementation limitations, this flag can be set/unset only on empty
files.
d
no dump, makes sense with 3rd party tools like
dump(8), on BTRFS the attribute can be set/unset on no other special
handling is done
D
synchronous directory updates, for more details
search open(2) for O_SYNC and O_DSYNC
i
immutable, no file data and metadata changes
allowed even to the root user as long as this attribute is set (obviously the
exception is unsetting the attribute)
S
synchronous updates, for more details search
open(2) for O_SYNC and O_DSYNC
X
no compression, permanently turn off compression
on the given file, other compression mount options will not affect that
When set on a directory, all newly created files will inherit this
attribute.
No other attributes are supported. For the complete list please refer to the
chattr(1) manual page.
CONTROL DEVICE¶
There’s a character special device /dev/btrfs-control with major and minor numbers 10 and 234 (the device can be found under the misc category).$ ls -l /dev/btrfs-control crw------- 1 root root 10, 234 Jan 1 12:00 /dev/btrfs-control
•scan devices for btrfs filesytem (ie. to let
multi-device filesystems mount automatically) and register them with the
kernel module
•similar to scan, but also wait until the device
scanning process is finished for a given filesystem
•get the supported features (can be also found
under /sys/fs/btrfs/features)
The device is usually created by ..., but can be created manually:
# mknod --mode=600 c 10 234 /dev/btrfs-control
SEE ALSO¶
acl(5), btrfs(8), chattr(1), fstrim(8), ioctl(2), mkfs.btrfs(8), mount(8)10/02/2016 | Btrfs v4.7.3 |